On a digital versatile disk (DVD), digital data can be recorded at a recording density which is about six times that of a compact disk (CD). The DVD is known as an information recording medium, i.e., a so-called optical disk on which large-volume digital data such as movie data or music data can be written.
In recent years, since the amount of information of information to be recorded increases, an information recording medium having a larger capacity is desired.
In order to increase the capacity of the information recording medium of the optical disk, the information recording density must be increased. In general, this can be, in a data write or read state, realized by decreasing a spot diameter of a laser beam radiated on the optical disk. In order to decrease the spot diameter of the beam, the wavelength of the laser beam may be further shortened, and the numerical aperture (NA) of an objective lens may be increased. For a DVD, a light source having a wavelength of 660 nm and an objective lens having an NA of 0.6 are used. Furthermore, for example, by using a blue laser beam having a wavelength of 405 nm and an objective lens having an NA of 0.85, information can be recorded at a recording density which is five times that of a current DVD.
In addition to shorten the wavelength of the laser beam by using the blue laser or the like, a technique of forming a plurality of recording layers in one optical disk is in development, in order to further increase the recording density. For example, if an optical disk having two recording layers can be obtained, in addition to the shortening of the wavelength of the laser beam and use of an objective length having a large NA, the recording density is about ten times that of the DVD having one recording layer.
However, in an optical disk apparatus having a blue laser as a light source, the margin of an optical power for reproduction in the blue laser is very small, quantum noise of the optical source poses a problem.
For example, in a conventional optical disk apparatus described in Japanese Unexamined Patent Publication No. 2000-195086, an optical pickup device in which an intensity filter serving as optical beam transmissive adjusting means is installed such that the optical beam transmissive adjusting means can be almost vertically inserted into and removed from a path of a laser beam is disclosed. In this optical disk apparatus, the intensity filter is inserted into the path of the laser beam during reproduction and is removed from a path of an emitted beam during recording. In this manner, for example, quantum noise of the semiconductor laser can be kept low, and high-quality reproduction can be performed.
However, in the optical disk apparatus, the intensity filter is arranged such that the intensity filter can be vertically and linearly removed from or inserted into the path of the laser beam. For this reason, a space to move the intensity filter is necessary. As a result, the optical pickup device disadvantageously increases in size.
In order to solve the problem, it may be possible to use another optical pickup device in which an intensity filter moving on an optical path by rotation is arranged in place of the intensity filter linearly moving on an optical axis. Referring to FIGS. 11 to 13, the optical pickup device will be described below.
FIG. 11 shows a configuration of a conventional optical pickup device. When a GaN-based blue-color light-emitting semiconductor laser beam source 41 radiates a blue optical beam, the optical beam enters to an optical beam transmissive adjusting means 200. The optical beam transmissive adjusting means 200 is pivoted to a predetermined position depending on a data read state from an optical disk 50 or a data write state on the optical disk 50, and the position of the intensity filter is adjusted. The optical beam transmitted through the optical beam transmissive adjusting means 200 is reflected by a beam splitter 42, collimated by a collimator lens 43, reflected by a mirror 44, and focused on the optical disk 50 through an objective lens 45.
In the data read state, the focused optical beam is reflected by a recording layer of the optical disk 50, reaches the beam splitter 42 through a reverse path, passes through the beam splitter 42, and then enters to a photodiode 48 through a multi-lens 47. The photodiode 48 is a so-called photodetector. The photodiode 48 outputs an electric signal on the basis of the position and the intensity of the incident beam. On the basis of the electric signal, data is reproduced.
On the other hand, in the data write state, an optical spot is formed on an information layer by the focused optical beam. As a result, a state of a recording layer at a portion where the optical spot is formed, e.g., a crystal state changes depending on data to be written. In this manner, in the optical disk 50, data is written as a change of states of the recording layers.
FIG. 12(a) is a perspective view obtained when the optical beam passes through the optical filter of the optical beam transmissive adjusting means 200 and corresponds to an arrangement in the data read state. The optical beam transmissive adjusting means 200 has a transmissive element 201. The transmissive element 201 has a pair of first parallel planes including two planes parallel to each other and a pair of second parallel planes including two planes parallel to each other. On at least one plane of the pair of first parallel planes, an optical filter 201a which attenuates optical power of the transmitting optical beam through the transmissive element 201 is applied. The optical beam transmissive adjusting means 200 includes a support member 104 which supports the transmissive element 201 and a rotational drive unit 105 which rotationally drives the transmissive element 201 about a rotating shaft 103. The support member 104 supports the rotating shaft 103 rotatably such that the transmissive element 201 can be rotated about the rotating shaft 103 which is parallel to the four planes constituting the pair of first parallel planes and the pair of second parallel planes of the transmissive element 201. The rotational drive unit 105 rotationally drives the transmissive element 201 about the rotating shaft 103.
FIG. 12(b) is a perspective view obtained when the optical beam do not transmit through the optical filter 201a of the optical beam transmissive adjusting means 200 and corresponds to an arrangement in the data write state. The optical beam transmissive adjusting means 200 can switch a position where the optical beam passes through the pair of first parallel planes of the transmissive element 201 and a position where the optical beam passes through the pair of second parallel planes through rotational drive about the rotating shaft 103. In this manner, the case in which the optical beam passes through the optical filter 201a and the case in which the optical beam does not pass through the optical filter 201a can be switched to each other. When the optical beam passes through the optical filter 201a, optical power can be suppressed to a low level in comparison with the case in which the optical beam does not pass through the optical filter 201a. 